1. Technical Field
The present invention relates to the field of telecommunications and, more particularly, to pre-connection telecommunication activities.
2. Description of the Related Art
Before a calling telephony device can call a receiving telephony device, a number of pre-connection activities can occur. In one such pre-connection activity, a calling device can signify that a facsimile (fax) message, as opposed to a voice communication, is being sent to the receiving device. Whenever this receiving device is capable of accepting the fax message, the receiving device may initialize a fax connection instead of generating a ringing sound (to indicate that a receiving party is being called). Since receiving parties often have a telephone receiver and a fax machine connected to the same telephone line, some mechanism providing a means for a receiving device to differentiate between attempted voice communications and incoming fax communications can be beneficial.
One incoming call identification method has been specified within the T.30 and T.4 protocols, which are predominantly utilized and recommended by the International Telecommunications Union (ITU). According to these protocols, an originating device can indicate a fax call with a fax calling tone (CNG), which is a short periodic tone that begins immediately after a phone number is dialed. This tone, also called a FAX CNG signal, is specified as an 1,100 Hz signal continuing for 0.5 seconds and repeated every 3 seconds. Consequently, telephony networks generally allow for the conveyance of a FAX CNG signal from a calling device to a receiving device at any point before the establishment of a connection.
Telephony companies (using circuit-switched technology) reserve a specified amount of bandwidth between an originating device and a receiving device for the duration of a telephone connection. The bandwidth normally reserved for one telephone voice channel (DSO) can accommodate a transmission rate of 64 kbps. This reserved bandwidth allows both the originating device and the receiving device to convey voice band (0–3,400 Hz) signals to one another.
While pre-connection signaling also occurs within the voice band (0–3,400 Hz), the entire frequency range of the voice band may not be available until after a connection is established. For example, filters are typically placed within telephony networks which prevent selected frequencies from propagating through the network. Such filters, however, do allow FAX CNG signals to pass. Hence, a segment of the voice band including the 1,100 Hz frequency is available for pre-connection signaling. Presently, other than the FAX CNG signal, a pre-connection frequency range including the 1,100 Hz frequency and available surrounding frequencies remains unused.